Conventional design of external line repeaters has previously utilized a transformer hybrid for isolation between the exchange and the main station. Effective galvanic isolation and good properties regarding balance impedance etc. have thus been obtained. However, these conventional solutions require large space and are furthermore costly, since such a transformer hybrid requires considerable manual work and costs for mechanical attention. For this reason it has been attempted to replace the transformer with electronic components not requiring as much space and affording cheaper solutions. Optocouplers having good isolation ability and including light-emitting diodes (LED) and phototransistors have appeared to be a conceivable replacement for the transformer in these cases. For example, the article "Octocoupler-Based Extension-Line Circuit for Electronic PABX's", IEEE Journal of Solid State Circuits Vol SC-13 No 1, Feb. 1978 describes an external line repeater containing optocouplers for isolation between two-wire side and four-wire side.